This project will study changes in responses of B lymphocytes with aging. Specifically we will investigate changes in the mouse lymphoid system which result in the appearance of B lymphocytes which can be activated by anti-immunoglobulin (Ig) reagents. Special emphasis will be given to the possible role of T lymphocytes in this process. In addition, detailed in vitro studies of the molecular requirements for anti-Ig induced B lymphocyte activation will be done using specifically purified reagents, (e.g. (F(ab1)2 and Fab fragments of IgG molecules, heavy chain monospecific antisera, C3 and C3b components of complement, etc.).